Open magenetic resonance imaging (mri) magnet system

ABSTRACT

The invention relates to an open magnetic resonance imaging (MRI) magnet system ( 1 ) for use in a medical imaging system. The open MRI magnet system has two main coil units which are accomodated, at some distance from each other, in a first housing ( 2 ) and in a second housing ( 3 ), respectively. Between the two housings, an imaging volume ( 6 ) is present wherein a patient to be examined is placed. A gradient coil unit ( 9, 10 ) facing the imaging volume is present near a side of each of the two housings. Functional connections of the gradient coil units ( 9, 10 ), such as electrical power supply lines ( 13, 14 ) and cooling channels ( 15, 16 ) are provided in a central passage ( 4, 5 ) which is present in each of the two housings. As a result, these functional connections do not reduce the space in the imaging volume available for the patient. The central passages and the electrical power supply lines provided therein are parallel to the direction of the main magnetic field of the open MRI magnet system, so that the Lorentz forces exerted on the power lines are limited.

The invention relates to an open magnetic resonance imaging (MRI) magnetsystem comprising a first mainly ring-shaped main coil unit and a secondmainly ring-shaped main coil unit extending parallel and at somedistance from each other and defining therebetween an imaging volume, afirst housing and a second housing which accommodate respectively thefirst main coil unit and the second main coil unit, at least the firsthousing having a central passage, the open MRI magnet system furthercomprising a first gradient coil unit and a second gradient coil unitassociated with respectively the first main coil unit and the secondmain coil unit, the first gradient coil unit and the second gradientcoil unit respectively being positioned between the associated main coilunit and the imaging volume, each gradient coil unit being provided withfunctional connections.

The invention further relates to a medical imaging system comprising anopen magnetic resonance imaging (MRI) magnet system.

An open MRI magnet system and a medical imaging system as mentioned inthe opening paragraphs are known from European patent applicationEP-A1-0 770 883. In this patent document an open MRI magnet system isdescribed having two coaxially aligned toroidal-shaped housings whicheach accommodate an annular-shaped superconductive main coil unitimmerged in a cryogenic fluid. A generally non-permanently magnetizedferromagnetic ring is located at an inner corner of each of theassociated housings. Centrally between the two housings a sphericalimaging volume is present. In order to be able to create an image of apatient for medical-diagnostical reasons, this patient should be presentwithin this imaging volume. The open structure of such an open MRImagnet system prevents or reduces claustrophobia feelings of patientswhich are more likely to occur when so-called closed magnet systemshaving an imaging volume within a single, tubular-shaped coil assemblyare being used. Whereas the two main coil units create a magnetic fieldbetween them within the imaging volume which is necessary to be able toproduce an image of an object such as a patient within the imagingvolume, the generally non-permanently magnetized ferromagnetic rings arepresent for further homogenization of the magnetic field within theimaging volume.

Usually, such an open MRI magnet system comprises gradient coil unitswhich are provided with functional connections, e.g. electricalconnections. The gradient coil units serve in general for generating aswitchable magnetic field, which is more or less linear in space, withinthe imaging volume as will be known to those skilled in the art.Usually, the functional connections are provided in the imaging volume,as a result of which the space available for the patient is reduced. Inthis respect it should be realized that by increasing the distancebetween the main coil units in order to increase the available space inthe imaging volume, also the costs related with the open MRI magnetsystem increase drastically.

It is an object of the invention to provide an open MRI magnet systemand a medical imaging system of the kinds mentioned in the openingparagraphs in which the functional connections are provided in such away that the available space for an object in general or, morespecifically, the available patient space for manipulation of a patientis optimized without the necessity to increase the distance between themain coil units.

In order to achieve this object, an open MRI magnet system according tothe invention is characterized in that at least one of the functionalconnections of the first gradient coil unit extends through the centralpassage of the first housing.

In order to achieve this object, a medical imaging system according tothe invention is characterized in that the open MRI magnet system usedtherein is an open MRI magnet system in accordance with the invention.

In this way the at least one functional connection for the firstgradient coil unit does not have to penetrate the space between the twohousings, thus leaving the maximum amount of space available for objectssuch as patients. This is advantageous both in respect of the availablespace and in respect of safety and acoustic noise which may be generatedby those functional connections.

According to a preferred embodiment of the invention the at least onefunctional connection for the first gradient coil unit comprises anelectrical connection for electrically energizing the first gradientcoil unit. In this way the electrical connection, for example embodiedas power cables, is directed parallel to the field lines of the maincoil unit. This means that during operation the Lorenz forces acting onthe electrical connection will be practically zero. In this manner theelectrical connection remains stationary during operation, which causesboth mechanical fatigue of the cable connectors and the need for theusual measures for reducing mechanical restraint and acoustic noise tobe reduced.

According to another preferred embodiment of the invention, the at leastone functional connection for the first gradient coil unit comprises acooling connection for conducting a cooling medium for the cooling ofthe first gradient coil unit. Apart from the above-mentioned advantageof the very efficient use of the available space, this gives theadditional advantage that the cooling medium, such as water or air, canapproach the first gradient coil unit centrally allowing homogeneouscooling of this gradient coil unit.

In order to be able to use short functional connections and to use theavailable space as efficiently as possible, the at least one functionalconnection for the first gradient coil unit is preferably connected tothe first gradient coil unit at a side of the first gradient coil unitfacing away from the imaging volume.

If the first gradient coil unit essentially shuts off the side of thecentral passage which is directed to the imaging volume, this firstgradient coil unit will be able to perform its task of generating aswitchable magnetic field, which is more or less linear in space, withinthe imaging volume in an even more optimal way.

For several reasons it can be very advantageous if the first gradientcoil unit comprises a further central passage having a diameter smallerthan 10 cm. Such a further central passage in the first gradient coilunit allows several useful applications. For example a laser beam can betransmitted through this further central passage, thus making itpossible to point to a certain location at a patient's body with a laserbeam. Another useful application relates to the mechanical manipulationof the second gradient coil unit using a cable or the like which extendsthrough the further central passage in the first gradient coil unit.

Preferably the central passage has a diameter smaller than 50 cm. Thediameter of the central passage can be relatively small since there isno need for transporting a patient therethrough to the imaging volume,as is the case in the open MRI magnet system according to EP 0 770 883A1. This means that the main coil unit is present relatively close tothe axis of the ring shape thereof, due to which fact it is lessdifficult to create a homogeneous magnetic field within the imagingvolume. The central passage is only present to accommodate functionalcomponents and its diameter can therefore be adapted to the functionalcomponents which the central passage should accommodate.

A constructionally very simple configuration is achieved when the firstgradient coil unit is essentially disc-shaped.

Alternatively the first gradient coil unit may be essentiallyconical-shaped.

The present invention will now be described, by way of example, withreference to the accompanying drawings in which:

FIG. 1 is a schematic diagram of a first preferred embodiment of an openMRI magnet system according to the invention,

FIG. 2 is a schematic diagram of a second preferred embodiment of anopen MRI magnet system according to invention.

FIG. 1 schematically shows a first preferred embodiment of an open MRImagnet system 1 according to the invention. The open MRI magnet system 1is a part of a medical imaging system according to the invention. Theremaining parts of the medical imaging system, including a frame, apatient support unit, and a control unit, are not shown in FIG. 1 andmay be of a usual kind known to the skilled person. The open MRI magnet1 is mainly mirror-symmetrical, both horizontally and vertically, andcomprises an upper housing 2 and a lower housing 3. The housings 2, 3are mainly ring-shaped and each have a central cylindrical passage 4, 5having a diameter of approximately 30 cm. Between the housings 2 and 3 aspherical imaging volume 6 is present having a diameter of about 40 cm.An object such as a patient can be placed and moved within the imagingvolume 6 between the two housings 2, 3 in order to be able to create,with the aid of the open MRI magnet system 1, an image of the object.For this purpose the open MRI magnet system 1 comprises a number of coilunits in order to create a suitable magnetic field within the imagingvolume 6. Within each of the housings 2, 3 a main coil unit is present,which main coil units are not shown in FIG. 1. On the sides facing eachother, the housings 2, 3 are each provided with a disc-shaped recess 7,8. The cylindrical passages 4, 5 are in open connection with thedisc-shaped recesses 7, 8. Within each disc-shaped recess 7, 8 agradient coil unit 9, 10 and a RF coil unit 11, 12 are present. Thefunctions of the main coil units, the gradient coil units 9, 10 and theRF coil units 11, 12 are known to the person skilled in the art and willnot be explained any further. Whereas the main coil units generate apermanent main magnetic field, the gradient coil units 9, 10 areelectrically energized in order to create gradients of the main magneticfield. For this purpose, for each gradient coil unit 9, 10 an electricalcable 13, 14 is connected to the associated gradient coil unit 9, 10.The electrical cables 13, 14 are connected to the sides of the gradientcoil units 9, 10 facing away from the imaging volume 6 and extend fromthese points through the cylindrical passages 4, 5 parallel to thecentral axis thereof. Due to this configuration the electrical cablesrun parallel to the main magnetic field lines, causing the Lorenzforces, exerted on the electrical cables during operation, to bepractically zero. This results in a reduction of the acoustic noise andless mechanical fatigue of the electrical cables 13, 14. Besides theelectrical cables 13, 14 also cooling channels 15, 16 extend essentiallycoaxially through the cylindrical passages 4, 5 parallel to the magneticaxis. Through these cooling channels 15, 16 a cooling medium, such aswater, is transported to and from the gradient coil units 9, 10 forcooling purposes. Also the cooling channels 15, 16 connect to theassociated gradient coil units 9, 10 at the side of the gradient coilunits 9, 10 facing away from the imaging volume 6. Since none of thecables 13, 14 and channels 15, 16 penetrate the imaging volume 6, themaximum amount of volume will be available for a patient, thus allowinga relatively limited distance between the housings 2 and 3. In casu thisdistance is about 45 cm.

The second preferred embodiment of an open MRI magnet system 21 inaccordance with the invention as shown in FIG. 2 corresponds to acertain extent to that of the open MRI magnet system 1 of FIG. 1. Likethe open MRI magnet system 1, the open MRI magnet system 21 comprises anupper housing 22 and a lower housing 23 each having a cylindricalpassage 24, 25. Between the housings 22, 23 an imaging volume 26 ispresent in which a patient can be placed. Within each of the housings22, 23 a main coil unit, which is not shown, is present. Furthermore theopen MRI magnet system 21 is provided with two gradient coil units 29,30 and two RF coil units 31, 32 which mainly surround the gradient coilunits 29, 30. The assemblies of the gradient coil units 29, 30 with theRF coil units 31, 32 are conically shaped and, in order to be able toaccommodate these assemblies, the housings 2, 3 are each provided with aconically shaped recess 27, 28 at the sides of the housings 22, 23facing each other. Electrical cables 33, 34 and cooling channels 35, 36are connected to the gradient coil units 29, 30 at the top of theconical shape thereof. Just like disc-shaped gradient coil units 9, 10of the open MRI magnet system 1, conical-shaped gradient coil units 29,30 shut off the central cylindrical passage 24, 25 at the side of theimaging volume 26.

Although the invention has been described referring to open MRI magnetsystems in FIGS. 1 and 2 having a horizontal orientation with a verticalmagnetic field orientation, it should be understood that also MRI magnetsystems having a vertical orientation with a horizontal magnetic fieldorientation are included within the scope of the invention.

Although not illustrated in FIGS. 1 and 2, it is noted that it ispossible within the scope of the invention to use the central passage ofat least one of the two housings also as a feed-through for electricalRF cables for energizing RF coil units 11, 12. Within this passage alsoshim irons or shim coils could be accommodated. The central passagefurthermore enables unique attachment features for the gradient coilunits to be attached to the associated housing. For this purposeresilient members may be present between the gradient coil unit and theassociated housing. Finally it is noted that the central passage canalso be used as a cooling conduit itself for the open MRI magnet systemor more specifically for the gradient coil units, or that it mayaccommodate cooling ducts for cooling air.

1. An open magnetic resonance imaging (MRI) magnet system comprising afirst mainly ring-shaped main coil unit and a second mainly ring-shapedmain coil unit extending parallel and at some distance from each otherand defining therebetween an imaging volume, a first housing and asecond housing which accommodate respectively the first main coil unitand the second main coil unit, at least the first housing having acentral passage, the open MRI magnet system further comprising a firstgradient coil unit and a second gradient coil unit associatedrespectively with the first main coil unit and the second main coilunit, the first gradient coil unit and the second gradient coil unitrespectively being positioned between the associated main coil unit andthe imaging volume, each gradient coil unit being provided withfunctional connections, characterized in that at least one of thefunctional connections of the first gradient coil unit extends throughthe central passage of the first housing.
 2. An open MRI magnet systemas claimed in claim 1, characterized in that the at least one functionalconnection of the first gradient coil unit comprises an electricalconnection for electrically energizing the first gradient coil unit. 3.An open MRI magnet system as claimed in claim 1, characterized in thatthe at least one functional connection of the first gradient coil unitcomprises a cooling connection for conducting a cooling medium for thecooling of the first gradient coil unit.
 4. An open MRI magnet system asclaimed in claim 1, characterized in that the at least one functionalconnection of the first gradient coil unit is connected to the firstgradient coil unit at a side of the first gradient coil unit facing awayfrom the imaging volume.
 5. An open MRI magnet system as claimed inclaim 1, characterized in that the first gradient coil unit essentiallyshuts off the side of the central passage directed to the imagingvolume.
 6. An open MRI magnet system as claimed in claim 5,characterized in that the first gradient coil unit comprises a furthercentral passage having a diameter smaller than 10 cm.
 7. An open MRImagnet system as claimed in claim 1, characterized in that the centralpassage has a diameter smaller than 50 cm.
 8. An open MRI magnet systemas claimed in claim 1, characterized in that the first gradient coilunit is essentially disc-shaped.
 9. An open MRI magnet system as claimedin claim 1, characterized in that the first gradient coil unit isessentially conical-shaped.
 10. A medical imaging system comprising anopen magnetic resonance imaging (MRI) magnet system, characterized inthat the open MRI magnet system is an open MRI magnet system as claimedin claim 1.